Project Summary Tobacco use remains a leading cause of preventable premature death in the United States, especially for racial/ethnic minorities and low-income populations. Public and multiunit housing are closely tied to tobacco- related disparities as vulnerable populations are most likely to live in multiunit housing with the highest rates of smoking and secondhand smoke exposure, while also the least likely to have access to smoke-free home environments. For example, New York City Housing Authority (NYCHA) residents smoke at higher rates, report higher secondhand smoke exposure from an outside source and are surrounded by high density tobacco retail environments compared to other New Yorkers. Public housing is at the forefront of the smoke-free policy movement. In 2016, HUD mandated that all housing authorities adopt smoke-free policies, which affects all 3,300 housing authorities and 1.2 million households nationwide. NYCHA is the largest public housing provider in the nation; it houses more than 400,000 people in 326 public housing developments spanning over 2,400 buildings citywide. As of July 2018, all NYCHA housing units adopted a smoke-free policy affecting all properties. The implementation of this policy, the largest of its kind, represents a critical opportunity to examine and reduce public housing-related tobacco hazards. To date, no randomized controlled trials (RCT) have been conducted to test the effects of compliance strategies on resident smoking behavior and secondhand smoke exposure. The proposed design will be a borough-stratified, four-arm, factorial-design, cluster RCT that will target 128 randomly selected buildings (32 buildings per arm) in separate NYCHA developments. We will recruit and follow 8 randomly selected residents stratified by smoking status- 4 smokers and 4 non-smokers- per building (n=1024) into four arms: (1) relocation/cessation, (2) resident endorsement, (3) relocation/ cessation plus resident endorsement and (4)the standard approach (256 participants per arm). Outcomes will be assessed longitudinally using salivary cotinine and resident surveys to measure smoking behaviors and secondhand smoke exposures at 4 time points: baseline, 2, 6 and 12 months post-intervention. Qualitative data will also be collected via key informant interviews, focus groups and sensory observations of buildings. This mixed methods, pre-post with comparison RCT design overseen by a highly experienced, interdisciplinary team in partnership with NYCHA and a stakeholder advisory board. Aims include to experimentally test if policy compliance interventions reduce personal smoking behavior (Aim 1); reduce secondhand smoke exposure (Aim 2), and determine if the tobacco retail environment surrounding each building moderates the relationship between compliance interventions and smoking-related outcomes (Aim 3). This comprehensive, community engaged and social-ecologically informed RCT supports the design and implementation of interventions to optimize smoke-free housing compliance for populations with high burdens of tobacco-related disparities.